Optical sensor with chemically reactive surface

ABSTRACT

An improved optical sensor and methods for measuring the presence of various materials or constituents in a fluid sample uses reactive material(s) in a fluid environment. The reactive materials have optical properties that change in the presence of a target material that may be present in the environment. An optical emitter generates light that is directed to the reactive materials, and one or more optical detectors receive reflected light from one or more interfaces in the optical path between the emitter and the detector(s), one or more of the interfaces having a reactive material. The reactive material(s), emitter(s), and detector(s) are selected based on the desired target material to be sensed.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under U.S. Army contractnos. DAAE30-02-C-1062, DAAE30-03-C-1075, W31P4Q-05-C-R100, andW31P4Q-06-C-0317. The Government may have certain rights to thisinvention.

FIELD OF THE INVENTION

The present invention generally relates to an optical sensor for sensingthe presence of a chemical substance in a fluid sample. Morespecifically, the invention relates to measuring optical characteristicsof a chemically reactive surface that is exposed to the fluid sample.

BACKGROUND OF THE INVENTION

Numerous applications require the determination of the presence orabsence of one or more substances in a particular sample. In particular,several application require that the presence or absence of a materialin a fluid sample be detected. Such applications include, for example,determining or monitoring of emissions from stationary or mobile sourcesfor the presence of one or more substances in the emissions. It would bebeneficial to have a sensor for detecting the presence or absence ofmaterials in various samples that is both efficient and relativelyinexpensive. Furthermore, such a sensor should be reliable andrelatively fast in performing the analysis. Furthermore, many of theapplications requiring such sensors are mobile or remote applicationsthat do not have ready access to a power supply that is not a battery orother stored type of power supply. Accordingly, it would also bedesirable for sensors in such applications to use relatively low powerwhen performing analysis of the samples.

SUMMARY OF THE INVENTION

Embodiments disclosed herein address the above stated needs by providingapparatuses and methods for sensing the presence or absence of varioustarget materials in an environment being sampled.

In one aspect an optical sensor, is provided that comprises an opticalemitter mounted on a substrate; an optical detector mounted on thesubstrate adjacent to the optical emitter; and a reactive surfacelocated opposite the substrate, optical emitter and optical detector. Anarea of the reactive surface is exposed to a fluid and an opticalproperty of the reactive surface changes upon exposure to a targetmaterial when the target material is present in the fluid. The opticalemitter emits light onto the reactive surface, and the optical detectorreceives reflected light from the reactive surface and detects thechange in said optical property when the reactive surface is exposed tothe target material. A transparent window may be located opposite thesubstrate, and the reactive surface located on the transparent windowopposite the substrate. The optical emitter may be comprised of one ormore of a vertical cavity surface emitting laser, a light emittingdiode, and a laser diode. The optical emitter may also include first andsecond optical emitters that emit light having different opticalcharacteristics, with a change in the reflected light opticalcharacteristics indicating the reactive surface is exposed to the targetmaterial. The optical detector may be comprised of one or more of aphoto diode, a charge coupled device, and a PIN photo detector. Theoptical detector may also include a first and a second optical detector,the first optical detector receiving reflected light from a first areaof the reactive surface and the second optical detector receivingreflected light from a second area of the reactive surface, where thefirst area is not exposed to the fluid, and the second area is exposedto the fluid. The reactive surface may comprise a plurality of differentreactive materials, each reactive material having an optical propertythat changes in a unique manner relative to other of the reactivematerials when exposed to the target material.

Another aspect provides a method for determining the presence or absenceof a target material in an environment, comprising the steps of (a)providing an optical sensor having an optical emitter, optical detector,and a reactive surface located in an optical path between the opticalemitter and optical detector; (b) monitoring an optical characteristicof light that is reflected off of a first area of the reactive surface;(c) determining if the optical characteristic has changed, and (d)providing an indication that the target material is present in theenvironment when it is determined that the optical characteristic haschanged. The step of monitoring, in one embodiment, comprises (i)receiving light that is reflected from a first area of the reactivesurface at a first optical detector, the first optical detectorgenerating a first output; (ii) receiving light that is reflected from asecond area of the reactive surface at a second optical detector, thesecond optical detector generating a second output; and (iii) monitoringa ratio of the first and second outputs. The step of determining, in anembodiment, comprises (i) receiving light that is reflected from a firstarea of the reactive surface at a first optical detector; (ii) receivinglight that is reflected from a second area of the reactive surface at asecond optical detector; (iii) multiplying the first output by a scalingfactor; (iv) subtracting the multiplied first output from the secondoutput to obtain a difference output; (v) amplifying the differenceoutput by a predetermined gain; and (vi) monitoring the amplifieddifference output. The scaling factor may be determined based on nominalfirst and second outputs so as to provide the multiplied first outputthat is substantially equal to the nominal second output.

In still a further aspect, an optical sensor is provided that comprisesan optical emitter mounted on a substrate, an optical detector mountedon the substrate adjacent to the optical emitter, a transparent windowlocated opposite the substrate, and a reactive surface located in thetransparent window. The reactive surface has a reference surface areaand a signal surface area, the reference surface area being isolatedfrom an environment being tested and the signal surface area beingexposed to the environment being tested, and an optical property of thesignal surface changes upon exposure to a target material when thetarget material is present in the environment. The optical emitter emitslight onto the signal and reference surfaces, and the optical detectorreceives reflected light from the signal and reference surfaces anddetects the change in the optical property when the signal surface isexposed to the target material based on a difference between thereflected light from the signal and reference surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an optical sensor of an embodiment;

FIG. 2 is a block diagram illustration of the electronics of an opticalsensor of an embodiment;

FIG. 3 is a flow chart diagram illustrating the operational steps of anoptical sensor of an embodiment;

FIG. 4 is a flow chart diagram illustrating the operational steps of anoptical sensor of another embodiment;

FIG. 5 is an illustration of reactive material and a transparent windowof an embodiment;

FIG. 6 is an illustration of an embodiment of an optical sensor andreactive surface on a portion of a transparent window;

FIG. 7 is an illustration of an embodiment of an optical sensor havingmultiple reactive surfaces and optical detectors;

FIG. 8 is an illustration of areas of reactive materials on atransparent window for an embodiment;

FIG. 9 is an illustration of photo detectors and an emitter on asubstrate for an embodiment;

FIG. 10 is an illustration of an optical sensor and associated filtersfor an embodiment;

FIG. 11 is an illustration of an optical sensor having multiple reactivesurfaces and transparent windows for an embodiment;

FIG. 12 is an illustration of an optical sensor having a lens shapedwindow for an embodiment; and

FIG. 13 is an illustration of an optical sensor having a shapedreflector for an embodiment.

DETAILED DESCRIPTION

The present invention generally relates to an improved optical sensorand methods for measuring the presence of various materials orconstituents in a fluid sample using a reactive material in the fluidsample that changes optical properties in the presence of the reactivematerial in the fluid sample. Optical sensors described herein userelatively little power and may also be used to sense an array ofdifferent parameters. The sensors of the various embodiments discussedherein rely on detecting a variation in intensity of light reflectedonto one or more photo detectors from one or more interfaces in theoptical path between an emitter and the photo detector(s). There areseveral general phenomena that result in intensity modulation at thephoto detector(s). These phenomena in various embodiments are utilizedsensor alone and/or in combination. A first general phenomena is anabsorption change within the optical path, such as the amount of lightabsorbed by a sensitive material layer that varies due to, for example,pressure, temperature, and/or presence of a chemical, for example. As aresult, the light reflected back onto the photo detector(s) from theinterface changes in the presence of the desired target. A secondgeneral phenomena is an index of refraction change at one or more of theinterfaces in the optical path. This phenomena results in the amount oflight reflected back onto a photo detector that changes based on theindex of refraction of each layer at each interface in the optical path.As the index of refraction of the sensitive material varies due to, forexample, pressure, temperature, and/or presence of a chemical, the lightreflected back onto the photo detector from the interfaces on eitherside of the sensitive material may vary. A third general phenomena is apolarization change at one or more of the interfaces in the opticalpath. The polarization state of the light reflected back onto the photodetector may be made to depend on the effect the various layers have onthe polarization of the light from the source. Finally, a geometricchange at one or more of the interfaces may alter the amount of lightreflected back onto the photo detector. Such embodiments may include oneor more designed features that promotes a desired deformation of some orall regions of one or more interfaces in the optical path when a forceis applied (e.g. due to pressure, acceleration, temperature change,etc.) to the interface. This deformation can be designed to steer raysof light onto or off of the photo detector, thus varying the intensityof light hitting the detector. By utilizing these phenomena eithersingly or in combination, along with appropriate selection of sensingmaterial(s) or appropriate design of glass features this sensing conceptcan be used to sense a wide range of physical and chemical parameters.

With reference now to the drawing figures, various embodiments of thepresent invention are described. With reference to FIG. 1, an opticalsensor of one embodiment is illustrated. In this embodiment, an opticalsensor 20 has a housing 24 containing a substrate 28 photo detectors 32and an emitter 36. A transparent window 40 is located opposite thesubstrate 28 and associated photo detectors 32 and emitter 36. Areactive surface 44 is then interconnected with the transparent window40 on the side of the window 40 that is opposite the substrate 28 andphoto detectors 32 and emitter 36. The emitter 36 emits lightillustrated by dashed lines 48 in FIG. 1. The emitted light 48 strikesthe reactive surface 34 which creates reflected light 52 that isreflected back towards the substrate 28. In this embodiment, photodetectors 32 are positioned to receive the reflected light. Such anoptical sensor 20 may be inserted into a sample such that the reactivesurface 44 comes into contact with the different constituent elementspresent in the sample. The reactive surface 44 is selected such thatoptical characteristics of the surface change in the presence of certainpredetermined constituents in the sample. When the constituents arepresent in the sample, the reactive surface 44 changes in opticalcharacteristics, with these changes being picked up by the photodetectors 32 monitoring the reflected light 52. The signal from thephoto detectors is measured by associated circuitry which can identifythe signal change from the photo detectors and provide an indicationthat one or more of the relevant constituents are present in the fluidstream of the optical sensor 20.

In one embodiment, the emitter 36 is a light emitting diode that emitslight at a determined frequency. The light emitted from the emitter 36illuminates or reflects off the reactive material 44, and then a portionof the emitted optical energy is cast back upon the photo detectors,which in one embodiment may be photo diodes. While described as lightemitting diodes and photo diodes, it will be understood that the emitter36 and photo detectors 32 may be any appropriate emitter, includinglight emitting diodes, laser diodes, vertical cavity surface emittinglasers (VCSELs), among others. Furthermore, the photo detectors mayinclude other suitable detectors as well, such as photo diodes, chargecoupled devices, PIN photo detectors, among others. Furthermore, one ormore filters may be integrated into the optical sensor 20 between theemitter and/or photo detectors. Through the change of opticalcharacteristics of the reactive material of the reactive surface 44, theintensity of the light striking the photo detectors 32 is also changed,resulting in a measurable signal change. The reactive surface 44 mayundergo a chemical, physical, or other change in the presence of one ormore substances that may be detected using the photo detector. Thereactive surface 44 may include reactive materials that may change inone of many possible optical characteristics in the presence of certainsubstances, such as, for example, absorption, index of refraction,fluorescence, and photo luminescence. The material that forms thereactive surface 44 can be any of a number of different types ofmaterials that undergo a change in the presence of one or moresubstances, such reactive materials may include chemically responsivethin films such as metal oxides, chemiluminecent dyes, polymer or solgel immobilized dyes, colorimetric dyes, and/or Langmuir Blodgett films,to name a few. The reactive material may also include thermallyresponsive materials such as thermo-chromic dyes and polymers anddimensionally changing materials. Furthermore, the reactive surface 44may detect physical changes such as changes in the angle of the reactivesurface 44 relative to the photo detectors 32. Such a change in physicalangle may be the result of, for example, expansion or contraction of thereactive surface 44 and/or one or more physical changes that result in achange of the angle at which the reactive surface 44 is situatedrelative to the substrate 28. Such physically responsive reactivesurfaces may include structures such as mirrors, cantilevers, gratings,photoelectric materials, and others that may move, for example, byinertial forces, pressure or temperature changes, and/or inducedstrains.

Referring now to FIG. 2, a block diagram illustrating an optical sensorof an embodiment is illustrated. In this embodiment, a controller 60 isinterconnected with an emitter 36 and provides signals thereto thatcause the emitter 36 to emit light. The controller is also operablyinterconnected with a detector portion 32, which may contain one or morephoto detectors as illustrated in FIG. 1, for example. The photodetector portion 32 provides signals to the controller 60 that thecontroller may monitor for changes to indicate the presence of aparticular substance in a sample being analyzed. The controller 60 isalso interconnected with an interface 64 that may be used to provide anoutput indicating the signal changes received from the optical detector32, and may provide an indication to the controller 62 provide aparticular signal to the emitter 36 in order to start an analysis. Theinterface 64 may include a user interface including a graphical userinterface, or may be an interface with another system that includes theoptical detector as a component therein.

With reference to FIG. 3, a flow chart diagram illustrating theoperational steps used by a photo detector of one embodiment todetermine the presence or absence of a particular target material in thesample being analyzed. Initially, as indicated at block 100, the opticalsensor provides light to the reactive surface associated with theoptical sensor. Similarly as described with reference FIGS. 1 and 2, theoptical sensor may include a photo diode or any other type of opticalemitter, that provides light to a reactive surface associated with theoptical sensor. At block 104, light is received from the one or morephoto detectors that is reflected from the reactive surface. Asdescribed above, the photo detector may be, for example, a photo diodeor multiple photo diodes. Next, at block 108, a signal output from thephoto detectors is determined. At block 112, a reference output from thephoto detectors is determined. In this embodiment, the reactive surfaceincludes two different areas. A first area that is exposed to the samplebeing analyzed, and a second area that is not exposed to a sample beinganalyzed. In this embodiment, at block 108, the signal output isdetermined based upon the reflected light from the reactive surface thatis exposed to the material being analyzed, and the reference output ofblock 112 is determined from the portion of the reactive surface that isnot exposed to the sample being analyzed. At block 116, a ratio of thesignal and reference outputs is determined. By taking a ratio of thesignal to the reference, the effect of emitter intensity fluctuationsthat can occur as a result of power supply fluctuations, componentaging, or temperature effects may be removed. In this manner, changes inthe intensity of the emitter will affect the reference and signaloutputs proportionally, and change in the emitter output results in theratio remaining unchanged when the environment being sensed is constant.Thus, changes in the optically responsive sensing material(s) affect thesignal output, and thus the ratio will change, indicating sensing of thedesired target. Writing this embodiment as a formula, when there is achange in emitter output but the environment being sensed is constant:

$\begin{matrix}{\frac{S_{final}}{R_{final}} = {\frac{S_{initial} + {AS}_{initial}}{R_{initial} + {AR}_{initial}} = \frac{S_{initial}}{R_{initial}}}} & (1)\end{matrix}$Where S is the signal output, R is the reference output, and A is theresulting change (in %) on the reference and signal outputs due toemitter output power variations. But, if the environment being sensed ischanging:

$\frac{S_{final}}{R_{final}} = {\frac{S_{initial} + {BS}_{initial}}{R_{initial}} = {\frac{\left( {1 + B} \right)S_{initial}}{R_{initial}} \neq \frac{S_{initial}}{R_{initial}}}}$Where and B is the resulting change (in %) on the reference and signaloutputs due to a change in the sensing material.

Referring again to FIG. 3, at block 120 the ratio of the signal andreference outputs is monitored, and at block 124 it is determined ifthere is a change in the ratio. Changes in the ratio, as mentioned,indicates that the portion of the reactive surface that is exposed tothe sample being analyzed has been exposed to the desired target. Atblock 128, if it is determined that there has been a change in theratio, an indication is provided that the desired target is sensed. Ifno change in the ratio is detected at block 124, an indication that thedesired target is not sensed is provided at block 132. Furthermore, inembodiments using multiple different photo detectors, when such photodetectors have similar electrical and thermal properties, taking theratio of the signal and reference outputs also removes proportionalchanges in the detectors due to power and/or temperature fluctuations.However, it will be understood that similar detectors may not always beused for various applications.

With reference now to FIG. 4, a flow chart diagram illustrating theoperational steps used by a photo detector of another embodiment todetermine the presence or absence of a particular target material in thesample being analyzed. Initially, as indicated at block 150, the opticalsensor provides light to the reactive surface associated with theoptical sensor. Similarly as described above, the optical sensor mayinclude a photo diode or any other type of optical emitter, thatprovides light to a reactive surface associated with the optical sensor.At block 154, light is received at the one or more detectors that isreflected from the reactive surface. As described above, the detectormay be, for example, a photo diode or multiple photo diodes. Next, atblock 158, a signal output from the detector(s) is determined. At block162, a reference output from the detector(s) is determined. In thisembodiment, similar to the embodiment of FIG. 3, the reactive surfaceincludes two different areas. A first area that is exposed to the samplebeing analyzed, and a second area that is not exposed to a sample beinganalyzed, and the signal output is determined based upon the reflectedlight from the reactive surface that is exposed to the material beinganalyzed, and the reference output is determined from the portion of thereactive surface that is not exposed to the sample being analyzed. Atblock 166, a scaling factor is selected based on the signal andreference outputs. The scaling factor, in an embodiment, is selectedsuch that if the nominal reference output is multiplied by the scalingfactor, the resulting multiplied reference output would be substantiallyequal to the nominal signal output. The signal output and the referenceoutput may have nominal values that are different due to a variety ofreasons, such as different photo detector(s) that receive the signallight and the reference light, different distances and/or angles betweenthe signal and reference reactive surfaces and the photo detector(s),and environmental temperature differences that affect the detector andemitter operation.

At block 170, the reference output is multiplied by the scaling factorthat was selected at block 166. As will be understood, the scalingfactor may be greater or less than one, thus amplifying or attenuatingthe reference output as desired for the optical sensor. At block 174,the multiplied reference output is subtracted from the signal output,producing a difference output. In embodiments where the scaling factoris selected to produce multiplied reference output that is substantiallyequal to the signal output, the resulting nominal difference output iszero. Any deviations of the difference output that are thensubstantially different than zero may indicate that the target materialis sensed. The difference output in this embodiment is then amplified bya desired gain, as indicated at block 178. The gain may be selected toproduce an amplified difference output to amplify changes to the signaloutput that are different than any changes to the reference output, thusproviding an enhanced output that may be analyzed to determine if thetarget material is present. At block 182, the ratio of the amplifieddifference output and the reference output is determined. By taking aratio of the amplified difference output to the reference output, theeffect of emitter intensity fluctuations that can occur as a result ofpower supply fluctuations, component aging, and/or temperature effectsmay be reduced or removed. Changes in the intensity of the emitter willeffect the reference and signal outputs proportionally, and any changein the emitter output would then result in the ratio of the amplifieddifference output and the reference output remaining relativelyunchanged when the environment being sensed is constant.

Again, because the reference and signal are both proportional to theemitter output, the result of this difference is also proportional tothe emitter output, and thus the effects of the emitter power variationcan be removed from the result similarly as described above with respectto FIG. 3. A change in the signal output only, resulting from a changein the sensing material only, can then be amplified additionally,without amplifying the offset, and still be proportional to the emitterpower. When a change in the sensing material then occurs, the differenceis then equal to just the amplitude of the change due to the sensingmaterial. If this value is very small, it can then be amplified towhatever scale is desired. This final value is then divided by thereference value, to get a ratio that is unaffected by emitter powervariations. Or written simply:

$\left. \frac{G_{2}\left( {S_{initial} - {G_{1}R_{initial}} + {BS}_{initial}} \right)}{R_{initial}}\Rightarrow{{if}{\mspace{14mu}\;}\left( {{S_{initial} - {G_{1}R_{initial}}} = 0} \right)} \right. = \frac{G_{2}{BS}_{initial}}{R_{initial}}$and, when there is a change in emitter optical output:

$\frac{G_{2}\left( {{AS}_{initial} - {G_{1}{AR}_{initial}} + {BAS}_{initial}} \right)}{{AR}_{initial}} = \frac{G_{2}\left( {S_{initial} - {G_{1}R_{initial}} + {BS}_{initial}} \right)}{R_{initial}}$Where G1 is the multiplication of the reference to equal the signaloutput, and G2 is any additional amplification needed to change theoutput scale of the signal change. This is useful for in embodimentswhere the change in signal is very small compared to any signal offset.Removing the offset before adding additional amplification allows forgreater overall amplification and resolution of the signal outputchange. This calibration can be done using electronic circuitcomponents, or computationally.

Referring again to FIG. 4, at block 186 it is determined if there is achange in the ratio. As mentioned, changes in the ratio would indicatethat the portion of the reactive surface that is exposed to the samplebeing analyzed has been exposed to the desired target. At block 190, ifit is determined that there has been a change in the ratio, anindication is provided that the desired target is sensed. If no changein the ratio is detected at block 186, an indication that the desiredtarget is not sensed is provided at block 194. Furthermore, inembodiments using multiple different photo detectors, when such photodetectors have similar electrical and thermal properties, scalingfactors for the different detectors may be selected individually toproduce a nominal difference output that is zero, or close to zero. Theratio of the difference output and reference output also removesproportional changes in the detectors due to power and/or temperaturefluctuations.

As discussed above, the reactive surface is selected to react to adesired target in the sample being analyzed, and this reaction resultsin a change in an optical characteristic of light reflected from thereactive surface. With reference now to FIG. 5, the reactive surface 200of one embodiment is discussed in more detail. In this embodiment, thereactive surface 200 is located on a transparent window 40 as describedwith respect to FIG. 1. The reactive surface 200 includes a firstsurface area 204 that is exposed to the surrounding environment, and asecond area 208 that is not exposed to the outside environment. In thisembodiment, the second surface area 208 is encapsulated by an epoxy 212such that the reactive material of the second surface area 208 isinsulated from the surrounding environment. As will be understood,numerous other types of materials and configurations may be used toinsulate the second surface area 208 from the sample being tested, solong as the insulating material provides an adequate barrier between thesample being tested and the second surface area 208. For example, thesecond surface area 208 may be coated with a metal, metal oxide, sol gelderived material, glass material, as well as one or more types ofpolymer with insulative properties, to name but a few. Furthermore, someembodiments do not have a reactive surface with two separate surfaceareas, instead having only a single area with a reactive surface thatcovers a portion of the transparent window. Light reflected from thisportion of the transparent window may then be compared to reflectedlight from portions of the transparent window that do not have thereactive material. In the embodiment of FIG. 5, the reactive surface 200is relatively thin, such that exposure to the desired target to thefirst surface area 204 will result in light reflected off of theopposite side of the reactive surface will have a changed opticalproperty. The thickness of the reactive surface 200 is thus dependentupon the material of the reactive surface, and the desired target.Reactive materials for the reactive surface 200 are generally uniquematerials that exhibit a chemical reaction when in contact with thedesired target. Such reactive materials include metal oxides with anindex of refraction that exhibits a change in response to exposure tocertain chemicals. For example, a 2000 nm film of BaTiO₃ will changeindex detectably in response to humidity. Other example metal oxidesthat may be similarly used include, but are not limited to: WO₃, SnO₂,In₂O₃, and Al₂O₃.

FIG. 6 is an illustration of an optical detector 220 having a 2000 nmfilm of BaTiO₃ metal oxide as a reactive surface 224. The reactivesurface 224 is deposited on a portion of transparent window 228. Theemitter and detector hardware, in this embodiment, include an 850 nmVCSEL emitter 232, and several silicon based photo detectors 236. In theevent that the reactive surface 224 is exposed to the desired target,the desired target, in this embodiment, humidity (i.e. water molecules),reacts with the BaTiO₃ and changes the reactive surface 224 in both theintensity of the light reflected (index of refraction) and the directionof the light reflected (angle of reflection). The position and shape ofthe detectors 236 is determined based on the different opticalcharacteristics of the reflected light when the reactive surface 224 isexposed to the desired target and when the reactive surface 224 is notexposed to the desired target. The detectors 236, in this embodiment,are placed such that the reflected light position change maximizes thechange in total intensity on the detectors. If a region of the sensorwindow is left bare (without any oxide coating), as in FIG. 6, the lightreflecting from that region will not change in response to humidity, andwill be proportional to the emitter 232 output intensity, thus acting asa power reference. In other embodiments, the detectors are positionedappropriately such that multiple oxides can be used to sense multiplechemicals, with the same sensor target. Other metal oxides that may besimilarly used include, for example, WO₃, SnO₂, In₂O₃, and Al₂O₃.

While a metal oxide is illustrated in FIG. 6 as the reactive surface224, various other materials may also be used. For example, certainembodiments utilize polymers with entrapped dyes exhibiting acolorimetric change. More specifically, certain dyes, immobilized incertain polymers will exhibit a colorimetric change in response toexposure to certain chemicals. For example, the dye crystal violet,immobilized in the proton exchange polymer Nafion will exhibit a colorshift from yellow to blue when exposed to humidity. The emitter anddetector hardware used in such an embodiment may be a LED emitting at600 nm, and multiple silicon photo detectors. In the presence ofhumidity (water molecules) the polymer and dye will absorb the 600 nmlight, and decrease the optical signal hitting the detectors. Again, theposition of the detectors is determined such that the reflected lightmaximizes the change in total intensity on the detectors. Other dyesimmobilized in polymers that may be similarly used include, for example,methylene blue in polymethyl methacrylate (PMMA), methylene green ingelatin, and CoCl in polyvinyl alcohol (PVA).

In addition to composition of the reactive material, hardware featuresmay also be modified and selected based on the desired target for theoptical sensor. More specifically, The type of emitter may be selectedbased on properties of the reactive surface and the desired target.Emission wavelength, for example, may be selected to provide an enhancedchange in the light at the specified wavelength in the presence of thedesired target. Similarly, emitter type may be selected to provideenhanced light change from the reactive surface. Additionally, anoptical sensor may include one or more different emitter types based onproperties of the reactive surface(s) and the desired target(s). Suchemitter types may include, for example, LED, RCLED, edge emitting laserdiode, and VCSEL. Furthermore, the type of detector may be selected todetect expected changes in the reflected light in the presence of thedesired target, and one or more different types of detectors may be usedin a sensor. Such detectors may include, for example, silicon basedphoto detectors, a CCD or CCD array, photodiodes, photoresistors,phototransistors, thermal detectors (bolometers), 1D or 2D arrays,compound semiconductor-based photodetectors, andmetal-semiconductor-metal (MSM) detectors. In still further embodiments,the optical sensor includes dielectric or absorptive filters to enhancechanges in the reflected light. The distance of the reactive materialfrom the detectors may also be selected based on the characteristics ofthe light reflected from the reactive surface in the presence andabsence of the desired target. The material of the transparent windowmay also be selected to provide enhanced detection of the desiredtarget, and/or the reactive material may be placed only in certainregions on the window. In even further embodiments, reflectors and/ormirrors are included in the optical sensor that direct light todetectors. Some examples of such embodiments will be described infurther detail below.

While the embodiments of FIGS. 1-6 describe an optical sensor designedwith a single reactive material on the sensor window to detect theconcentration or the presence/absence of a single material in theenvironment, such as a chemical vapor or relative humidity, otherembodiments are capable of detecting the presence/absence of more thanone target material in the environment. Furthermore, in manyapplications, the reactive surface may react to the target material, aswell as other materials that are not of interest. For example SnO is anoxide material that reacts to many airborne materials including relativehumidity, temperature, the vapors of a number of volatile organiccompounds, and many other chemicals. Thus, a reactive surface havingmaterial based on SnO will react to all of these different materials inthe environment, and generate a signal. However, in such a case, thereis no way to determine if the sensor was exposed to, for example, watervapor or methanol vapor. In one embodiment, illustrated in FIG. 7,additional reactive surface regions may be included in a sensor 250,with different regions having different reactive materials. In thisembodiment, several different reactive surfaces 254 a, 254 b, 254 c, 254d, are located on the transparent window 258. Similarly as describedabove, the reactive surfaces 254 a-d, are placed on the transparentwindow 258 so as to be exposed to an environment where it is desired todetermine of one or more target materials are present. In thisembodiment, the optical sensor 250 includes an optical emitter 262, thatmay be any appropriate optical emitter such as described above. Emittedlight 266 from the optical emitter 262 reflects off of the reactivesurfaces 254, and the reflected light 270 is received at an array ofoptical detectors 274 a, 274 b, 274 c, and 274 d. The emitter 262 andphoto detectors 274 are mounted on a substrate 278. In this embodiment,the reactive materials 254 a-d are placed on the transparent window 258such that reflected light 270 from the reactive surfaces 254 a-d isdirected to respective photo detectors 274 a-d. In one embodiment, oneof the reactive surfaces 254, such as reactive surface 254 d, is areference surface, and thus the output from the associated photodetector 274 d is used as the reference output, with the outputs fromeach of the remaining photo detectors 274 a, 274 b, and 274 c, beingsignal outputs. When one or more reactive materials 254 a-c exhibit aresponse to materials in the environment of the sensor, these reactionsdiffer based on the presence/absence of different materials in theenvironment, and it is possible to use the electrical signal from therespective reactive material and photo detector pairs as inputs to apattern recognition algorithm. If the pattern recognition algorithm isproperly defined based on known responses, the overall sensor system canbe made to distinguish different chemicals or other inputs even if eachindividual reactive material/photo detector pair cannot be used to makesuch a determination.

While the embodiment illustrated in FIG. 7 has four regions that aremonitored, FIGS. 8-9 illustrate another embodiment in which a sensor hassix photo detector regions 282 a-f on the substrate 278, five reactivematerial regions 286 a-e on the transparent window 258, and onereference material region 290 on the transparent window 258. While theembodiment of FIGS. 7-9 include a reference material and associatedphoto detector for the reference material that provides a referenceoutput for power and temperature compensation purposes, otherembodiments do not have such reference elements because power andtemperature compensation is not necessary for using pattern recognitionto detect the presence/absence of target materials in the environment.

Referring still to FIGS. 8-9, photo detector regions 282 b through 282 fproduce variable photocurrents that depend on the makeup of theenvironment surrounding the reactive material regions 286 a-e at a giventime. These photocurrents can be adjusted to compensate for temperatureand power variations with the photocurrent generated by the referencedetector 282 a to yield compensated outputs for each reactive materialregions 286 a-e. The five reactive material regions 286 a-e are occupiedby five different reactive materials that all respond to, and responddifferently to, for example, four different chemicals of interest. Theprecise state of the reactive materials 286, and thus the five differentphotocurrents, in depend on the concentrations, CA, CB, CC, and CD, ofthe four chemicals of interest. It should be noted that the values forthe number of photodetectors, reactive material regions, and chemicalsof interest here are used only as an example for purposes of discussion,and the numbers needed for particular applications may vary. By exposingthe reactive material regions 286 to various controlled concentrationsof the four chemicals and recording the compensated outputs of the photodetectors 282 b-f for each known environmental condition, training datafor training the pattern recognition algorithm are obtained. Thetraining data set consists of a suitably sufficient number ofinput/output groups that can describe the nature of the behavior of theoptical sensor when exposed to the various combinations of chemicals ofinterest. In order to reduce the need for a human to define the natureof this relationship, various embodiments rely on a properly trainedpattern recognition algorithm to predict the concentrations of thechemicals of interest based on the amplitudes of the electrical signalsproduced by the photo detectors 282 a-f. A variety of patternrecognition algorithms can potentially be used with an optical sensor ofthe type described here. Examples include neural networks, fuzzy logicmodels, and hidden Markov models. In each case, the pattern recognitionalgorithm is treated as a “black box” mathematical model with a suitablylarge number of adjustable parameters. These adjustable parameters areadjusted through a “training” process that is typically unique to theparticular pattern recognition algorithm being used until, when thealgorithm is given inputs, which correspond to the outputs of the photodetectors the outputs of the model predict the concentrations of thechemicals of interest to a suitable level of accuracy.

Referring now to FIG. 10, an embodiment is illustrated that utilizesmultiple optical sources and filters. In this embodiment, an opticalsensor 300 includes a number of reactive surfaces 304 that are locatedon a transparent window 308. In this embodiment, multiple emitters 312are used to generate light 316 that is directed to the reactive surfaces304. The multiple emitters 312 may generate light having differentwavelengths, and one or more of the reactive surfaces may be selectedsuch that the change in optical properties of the reflected light isrelatively sensitive to the wavelength associated with the particularemitter. Similarly, the different emitters may produce light with otherdiffering properties, such as different polarizations, different anglesrelative to the reactive surfaces, and/or different intensity, to namebut a few. The light 316 reflects off of the reactive surfaces 304 andthe reflected light 320 is directed to multiple photo detectors 324.Additionally, in this embodiment, the optical sensor 300 includesseveral filters and/or coatings that are associated with differentcomponents. In the embodiment of FIG. 10, an anti-reflective coating 328is located on the transparent window 308. Such anti-reflective coatings328 are well known in optics and optical systems, and serves to reducethe reflection that is generates from the surface of the transparentwindow 308 that is opposite the reactive surfaces 304, thus reducing thenoise that may be present from the photo detectors 324 that may resultfrom light reflected from the lower surface of the transparent window308. Furthermore, in the embodiment illustrated in FIG. 10, several ofthe photo detectors also include thin film and/or absorption filters332. Such filters may further enhance the signal output from the photodetectors 324. Such filters 332 may include, for example, polymer filmssuch as PMMA or polystyrene-co-methyl cethaerylate (PSMMA) doped withspecific wavelength absorbing dyes or pigments, glasses doped withspecific wavelength absorbing dyes or pigments, controlled thicknessdielectric films, and solid films made from materials with specializedabsorption characteristics.

Referring now to FIG. 11, an illustration of another embodiment isillustrated. In this embodiment, an optical sensor 350 includes reactivesurfaces 354 similar to the reactive surfaces previously described, andalso includes a reactive surface 358 and a physical surface 362. Lightemitted from an emitter 374 reflects off of the reactive surfaces 354,358, 362, and is received at optical detectors 378. In this embodiment,a first transparent window 366 has the reactive surfaces 354, and asecond transparent window 370 has the reactive surfaces 358, 362. Inthis manner, the reactive surfaces 358, 362, may be protected from theenvironment external to the first transparent window 370. For example,the reactive surface 358 may be responsive to temperature differences,and the reactive surface 362 may be an inertial responsive sensing area,both of which do not require exposure to function, and thus areprotected from the environment. Conversely, the chemically and/orbiologically sensitive areas 354 are exposed to the environment, withoutcompromising the function of the optical sensor 350.

FIG. 12 illustrates a further embodiment in which an optical sensor 400includes a lens shaped window 404. In this embodiment, light emittedfrom an emitter 408 reflects off of the reactive surface 412, andtravels through lens shaped window 404 before being received at opticaldetectors 416. In this embodiment, the lens shaped window 404 guides thereflected light to the optical detectors 416 in order to focus orotherwise direct the light to the detectors 416 in a manner that may beoptimal for a particular application.

FIG. 13 illustrates another embodiment in which light may be guided toprovide an appropriate amount of light at the optical detectors. In theembodiment of FIG. 13, an optical sensor 450 includes a shaped reflector454 that reflects light from an emitter 458 to optical detectors 462. Areactive surface may be included on the shaped reflector 454 such that achange in the optical properties of the reflected light may be used todetect the presence or absence of one or more target materials in theenvironment being sampled. The embodiment of FIG. 13 may also include areactive surface on window 466, with the reactive surface beingtransparent at the wavelength of the optical emitter and/or thewavelength of a changed optical signal that may result from the presenceof one or more target materials in the environment being sampled.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. An optical sensor, comprising: an optical emitter mounted on asubstrate; an optical detector mounted on said substrate adjacent tosaid optical emitter; and a reactive surface located opposite saidsubstrate, optical emitter and optical detector, wherein an area of saidreactive surface is exposed to a fluid and an optical property of saidreactive surface changes upon exposure to a target material, whereinsaid optical emitter emits light onto said reactive surface, and saidoptical detector receives reflected light from said reactive surface andis operable to detect the change in said optical property when saidreactive surface is exposed to said target material, the change inoptical property being independent of an angle of incidence of saidemitted light onto said reactive surface and including at least one of achange in index of refraction, a change in optical absorption, a changein polarization of the reflected light, a change in fluorescence, and achanged optical property resulting from a change in geometry of saidreactive surface, and wherein said optical detector comprises a firstand a second optical detector, said first optical detector located onsaid substrate to receive reflected light from a first area of saidreactive surface and said second optical detector located on saidsubstrate to receive reflected light from a second area of said reactivesurface that is different than said first area.
 2. The optical sensor,as claimed in claim 1, further comprising: a transparent window locatedopposite said substrate, and wherein said reactive surface is located onsaid transparent window on a side of said transparent window that isopposite said substrate, and wherein said optical emitter emits lightthat travels through said transparent window, reflects off of saidreactive surface, travels back through said transparent window, and thereflected light is received at said optical detector.
 3. The opticalsensor, as claimed in claim 2, further comprising: an anti-reflectivecoating located on the side of said transparent window that is towardssaid substrate.
 4. The optical sensor, as claimed in 1 wherein saidoptical emitter is comprised of at least one of a vertical cavitysurface emitting laser, a light emitting diode, and a laser diode. 5.The optical sensor, as claimed in claim 1, wherein said optical emittercomprises a first and a second optical emitter, said first opticalemitter emitting light having a first optical characteristic, and saidsecond optical emitter emitting light having a second opticalcharacteristic that is different than said first optical characteristic.6. The optical sensor, as claimed in claim 5, wherein said first andsecond optical characteristics are selected from the group consisting ofwavelength, polarization, and intensity.
 7. The optical sensor, asclaimed in claim 1, wherein said optical detector is comprised of atleast one of a photo diode, a charge coupled device, and a PIN photodetector.
 8. The optical sensor, as claimed in claim 1, wherein saidfirst area of said reactive surface is not exposed to said fluid, andsaid second area of said reactive surface is exposed to said fluid. 9.An optical sensor, comprising: an optical emitter mounted on asubstrate; an optical detector mounted on said substrate adjacent tosaid optical emitter; and a reactive surface located opposite saidsubstrate, optical emitter and optical detector, wherein an area of saidreactive surface is exposed to a fluid and an optical property of saidreactive surface changes upon exposure to a target material when saidtarget material is present in said fluid, and wherein said opticalemitter emits light onto said reactive surface, and said opticaldetector receives reflected light from said reactive surface and isoperable to detect the change in said optical property when saidreactive surface is exposed to said target material; wherein saidoptical detector comprises a first and a second optical detector, saidfirst optical detector located on said substrate to receive reflectedlight from a first area of said reactive surface and said second opticaldetector located on said substrate to receive reflected light from asecond area of said reactive surface that is different then said firstarea; wherein said first area of said reactive surface is not exposed tosaid fluid, and said second area of said reactive surface is exposed tosaid fluid; and wherein said first optical detector generates areference output, and said second optical detector generates a signaloutput, and a ratio of said reference and signal outputs is used todetermine the presence or absence of said target material.
 10. Theoptical sensor, as claimed in claim 1, wherein said reactive surfacecomprises a plurality of different reactive materials, each of saidreactive materials having an optical property that changes in a uniquemanner relative to other of said reactive materials when exposed to saidtarget material.
 11. The optical sensor, as claimed in claim 1, furthercomprising at least one optical filter located in an optical pathbetween said optical emitter and said optical detector.
 12. A method fordetermining the presence or absence of a target material in anenvironment, comprising: providing an optical sensor having an opticalemitter, optical detector, and a reactive surface located in an opticalpath between said optical emitter and optical detector; monitoring anoptical characteristic of light that is reflected off of a first area ofsaid reactive surface; determining if said optical characteristic haschanged, the change in optical characteristic being independent of anangle of incidence of said emitted light onto said reactive surface andincluding at least one of a change in index of refraction, a change inoptical absorption, a change in polarization of the reflected light, achange in fluorescence, and a changed optical property resulting from achange in geometry of said reactive surface, and providing an indicationthat said target material is present in the environment when it isdetermined that said optical characteristic has changed, wherein saidstep of monitoring comprises: receiving light from a first area of saidreactive surface at a first optical detector, said first opticaldetector generating a first output based on the light received at thefirst optical detector; receiving light from a second area of saidreactive surface at a second optical detector, said second opticaldetector generating a second output based on the light received at thesecond optical detector; and monitoring a ratio of said first and secondoutputs.
 13. A method for determining the presence or absence of atarget material in an environment, comprising: providing an opticalsensor having an optical emitter, optical detector, and a reactivesurface located in an optical path between said optical emitter andoptical detector; monitoring an optical characteristic of light that isreflected off of a first area of said reactive surface; determining ifsaid optical characteristic has changed, and providing an indicationthat said target material is present in the environment when it isdetermined that said optical characteristic has changed; wherein saidstep of determining comprises: receiving light that is reflected from afirst area of said reactive surface at a first optical detector, saidfirst optical detector generating a first output based on the lightreceived at the first optical detector; receiving light that isreflected from a second area of said reactive surface at a secondoptical detector, said second optical detector generating a secondoutput based on the light received at the second optical detector; andmultiplying said first output by a scaling factor; subtracting saidmultiplied first output from said second output to obtain a differenceoutput; amplifying said difference output by a predetermined gain; andmonitoring said amplified difference output.
 14. The method, as claimedin claim 13, wherein said scaling factor is determined based on nominalfirst and second outputs so as to provide said multiplied first outputthat is substantially equal to said nominal second output.
 15. Anoptical sensor, comprising: an optical emitter mounted on a substrate;an optical detector mounted on said substrate adjacent to said opticalemitter; a transparent window located opposite said substrate; and areactive surface located on said transparent window on a side of saidtransparent window that is away from said substrate, said reactivesurface having a reference surface area and a signal surface area, saidreference surface area being isolated from an environment being testedand said signal surface area being exposed to the environment beingtested, wherein an optical property of said signal surface changes uponexposure to a target material when said target material is present inthe environment, and wherein said optical emitter emits light onto saidsignal and reference surfaces, and said optical detector receivesreflected light from said signal and reference surfaces and is operableto detect the change in said optical property when said signal surfaceis exposed to said target material based on a difference between thereflected light from the signal and reference surfaces.
 16. The opticalsensor, as claimed in claim 15, wherein said optical detector comprisesa first and a second optical detector, said first optical detectorlocated on said substrate to receive reflected light from said signalsurface area and said second optical detector located on said substrateto receive reflected light from said reference surface area.
 17. Theoptical sensor, as claimed in claim 15, wherein said reactive surfacecomprises a plurality of different reactive materials, each of saidreactive materials having an optical property that changes in a uniquemanner relative to other of said reactive materials when exposed to saidtarget material.
 18. The optical sensor, as claimed in claim 15, furthercomprising at least one optical filter located in an optical pathbetween said optical emitter and said optical detector.
 19. The opticalsensor, as claimed in claim 1, wherein said first optical detectorgenerates a reference output, and said second optical detector generatesa signal output, and a ratio of said reference and signal outputs isused to detennine the presence or absence of said target material.